1. Field of the Invention
The present invention relates to a fuel cell stack which comprises a fuel cell unit composed of a solid polymer ion exchange membrane interposed between an anode electrode and a cathode electrode, and separators for supporting the fuel cell unit interposed therebetween, the fuel cell units and the separators being stacked in the horizontal direction. The fuel cell stack is especially appropriate to be carried on a vehicle.
2. Description of the Related Art
For example, the solid polymer type fuel cell comprises a fuel cell unit including an anode electrode and a cathode electrode disposed opposingly on both sides of an electrolyte composed of a polymer ion exchange membrane (cation exchange membrane) respectively, the fuel cell unit being interposed between separators. Usually, the solid polymer type fuel cell is used as a fuel cell stack obtained by stacking a predetermined number of the fuel cell units.
In such a fuel cell stack, a fuel gas, i.e., a gas principally containing hydrogen (hereinafter referred to as “hydrogen-containing gas”) which is supplied to the anode electrode, hydrogen being converted into ion on the catalyst electrode, is moved toward the cathode electrode via the electrolyte which is appropriately humidified. The electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas such as a gas principally containing oxygen (hereinafter referred to as “oxygen-containing gas”) or air is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode, and thus water is produced.
In the fuel cell stack described above, an internal manifold is constructed in order to supply the fuel gas and the oxygen-containing gas (reaction gas) to the anode electrode and the cathode electrode of each of the stacked fuel cell units respectively. Specifically, the internal manifold includes a plurality of communication holes which are provided in an integrated manner to make communication with each of the fuel cell units and the separators which are stacked with each other. When the reaction gas is supplied to the supplying communication hole, the reaction gas is supplied in a dispersed manner to each of the fuel cell units, while the used reaction gas is integrally discharged to the discharging communication hole.
The reaction product water, which is generated on the electrode power-generating surface, tends to be introduced especially into the communication hole through which the oxygen-containing gas flows. Retained water exists in the communication hole in many cases. On the other hand, it is feared that any retained water is generated due to condensation of water vapor or the like in the communication hole through which the fuel gas flows. Therefore, the following inconvenience is pointed out. That is, the communication hole is reduced in cross sectional area or closed by the retained water, and the smooth flow of the reaction gas is prevented. As a result, the power generation performance is deteriorated.
In view of the above, for example, as disclosed in Japanese Laid-Open Patent Publication No. 8-138692, a fuel cell is known, in which hydrophilic coating films are provided for a fuel gas flow passage and an oxygen-containing gas flow passage formed on a stacking surface of a collector electrode. Specifically, as shown in FIG. 26, supply/discharge flow passages 2a, 2b for the fuel gas are formed to penetrate through both side portions of a collector electrode 1. Supply/discharge flow passages 3a, 3b are formed to penetrated through upper and lower portions of the collector electrode 1. A plurality of oxygen-containing gas flow passages 4, which are parallel to one another in the vertical direction, are provided linearly on the side of the power-generating surface of the collector electrode 1. A hydrophilic coating film 5 is formed for the oxygen-containing gas flow passage 4. A porous member 6 is arranged for the oxygen-containing gas supply/discharge flow passage 3b. 
In the arrangement as described above, when the water, which is produced on the side of the power-generating surface in accordance with the operation of the fuel cell, is introduced into the oxygen-containing gas flow passages 4, the product water humidifies the hydrophilic coating film 5 formed for the oxygen-containing gas flow passage 4. The product water flows vertically downwardly along the hydrophilic coating film 5 and its surface, and it is discharged from the oxygen-containing gas flow passage 4. Further, the product water is absorbed by the porous member 6 which is arranged for the oxygen-containing gas supply/discharge flow passage 3b. As a result, it is stated that the product water can be reliably discharged from the oxygen-containing gas flow passage 4.
However, in the case of the conventional technique described above, the oxygen-containing gas supply/discharge flow passages 3a, 3b are formed at the upper and lower portions of the collector electrode 1. Therefore, it is difficult to shorten the size of the entire fuel cell in the height direction. Especially, in the case of the use as a fuel cell stack to be carried on a vehicle, it is necessary to effectively utilize the space, e.g., under the floor of the automobile body. It is demanded to shorten the size of the entire fuel cell in the height direction as small as possible. However, the conventional technique described above involves such a problem that it is impossible to effectively respond to the demand.
Further, the oxygen-containing gas supply/discharge flow passages 3a, 3b are formed to be lengthy in the lateral direction at the upper and lower portions of the collector electrode 1. For this reason, in order to ensure the rigidity of the collector electrode 1, it is necessary to set a relatively large thickness of the collector electrode 1. Accordingly, the problem is pointed out that the size of the entire fuel cell stack in the stacking direction is lengthy.
When the size of the entire fuel cell stack in the stacking direction is lengthy, the oxygen-containing gas supply/discharge flow passage 3b becomes long in the stacking direction. The problem also arises that the product water or the like existing at the deep side is difficult to be discharged. Especially, when the fuel cell stack is used to be carried on the vehicle, it is feared that the vehicle runs in an inclined state, and the product water is retained at the deep portion of the oxygen-containing gas supply/discharge flow passage 3b. At that time, the problem arises that the power generation performance is deteriorated because the product water is not discharged.
A technique is disclosed in Japanese Laid-Open Patent Publication No. 10-284096, in which water droplets are prevented from invasion into the power-generating surface by providing a gas branch groove which extends downwardly from a gas inlet of a communication hole. However, when the gas branch groove is provided for the power-generating surface, the amount of gas, which is discharged without contributing to the power generation, is increased. Then, the problem arises such that the ratio of utilization of the reaction gas is lowered, resulting in the decrease in efficiency of the entire system.
A technique is disclosed in U.S. Pat. No. 4,968,566, in which water discharge ports are provided at lower four corner portions of a fuel cell, and the discharge ports are switched depending on a signal of an inclination sensor. However, the problem arises that the structure of the apparatus is considerably complicated.